REVIEW OF TEST METHODS AND CRITERIA FOR ASSESSING TASTE AND OFF-TASTE
DWI0821
February 2002

EXECUTIVE SUMMARY

OBJECTIVES

To review and critically appraise available test methods for
assessing the taste and off-taste in drinking water and where possible
propose alternatives or new technologies which may be applicable.

REASONS

Taste and odour problems in drinking water present a difficult
customer relations problem for water companies. A clear evaluation of
the nature of a taste and odour problem in drinking water represents
the first step to treatment and control measures. However, there are
particular problems associated with the application of qualitative and
quantitative sensory methods to assess taste and odour in drinking
water.

CONCLUSIONS

Taste and odour assessment using sensory methods is apparently a
basic and simple test which consumers can do themselves. However, it is
essentially a subjective assessment which has considerable uncertainty.
Published standard methods for the assessment of tastes and odours in
drinking water suffer from methodological and interpretational problems
and there are significant practical and resourcing problems associated
with applying some methods, including the European Standard method,
which has received criticism.

The SCA and CEN methods are not per se methods for investigating the
causes of tastes and odours but provide a means of assessing compliance
with regulatory standards based on the response of panellists to tastes
and odours in samples and diluted samples with respect to reference
samples. The use of quantitative taste and odour measurements using
threshold numbers are the main means of assessing compliance with
regulations and should not be expected to necessarily be of value in
dealing with customer complaints. Whilst threshold odour or taste
(flavour) numbers are a useful method for obtaining information on
taste or odour thresholds for specific chemicals, when applied in the
SCA and CEN methods they simply provide a subjective assessment of the
presence or absence of a flavour or odour to panellists. They do not
provide specific information relating to the causes of any tastes or
odours detected and cannot distinguish between effects caused by single
chemicals or by mixtures of different chemicals. Although sensory tests
based on threshold measurements are often referred to as
'quantitative', they should really be regarded as being
'semi-quantitative', particularly as the nature of the organoleptic
chemicals is usually unknown. Despite the limitations of the threshold
approach it can be applied as a relative measure by treatment plant
operators to monitor changes in water quality.

Taste and odour tests used in the USA adopts a higher sample testing
temperature than in the CEN and SCA methods. Whilst a move to revise
the SCA or CEN methods to incorporate a higher test temperature may
intuitively appear to lead to increased method sensitivity, this
requires further investigation. There is a need to evaluate how a
change in test temperature influences vapour phase concentrations for a
range of relevant odourous chemicals which may have widely differing
physicochemical properties. There does not appear to be any background
literature which demonstrates the actual benefits that a higher test
temperature provides. A decision to recommend a revision of current
test method temperatures should be based on an objective comparison of
practical vapour pressure improvements with sensory detection errors.
This would establish whether any real practical improvements result
from increases in CEN or SCA test method temperatures.

The Drinking Water Directive, Annex A, Part C, states that the taste
and odour of drinking water should be "acceptable to consumers and
subject to no abnormal change". However, the threshold tests used for
compliance monitoring are non-specific, semi-quantitative and
essentially subjective. These inherents weaknesses, coupled with the
lack of definition regarding what is acceptable to consumers undermines
the validity of taste and odour compliance monitoring.

Threshold measurements do not take account of the offensiveness of
tastes or odours or of the fact that different chemicals may have
different dose responses, that these are non- linear and that different
panellists may have widely differing personal sensitivities to
chemicals. The inherent lack of precision in sample screening using
threshold numbers limits their utility. Furthermore, the fact that
compliance with statutory regulations is possible if 50% of the
panellists cannot perceive a taste or odour in a sample diluted with
three volumes of reference water is unlikely to reassure consumers,
particularly if they fall within the more sensitive 50% of the
population.

Consequently, compliance with the requirements of threshold tests
does not necessarily provide guarantees that customers will not find
drinking water unobjectionable in terms of taste and odour.

Some investigation as to how standard methods cope in practice with
a wide range of substances which cause tastes or odours, particularly
volatile chemicals, is required. Also, there is a need to incorporate a
satisfactory protocol to select and 'calibrate' the sensitivity of
panellists and to monitor their performance within current standard
threshold methods. Addressing this may reduce some of the subjectivity
of the methods. At present there do not appear to be any better sensory
methods available to measure the compliance with dilution-based
regulatory limits. There is certainly scope for investigating to what
extent meeting a 'quantitative' standard for taste and odour actually
ensures meeting consumers perceptions of acceptability and what
proportion of the population are likely to be satisfied.

In order to investigate the sources of taste and odour problems in
drinking water and to identify the chemical or microbiological causes
more elaborate methods such as Flavour Profile Analysis and analytical
screening need to be considered. The output of such testing is useful
for process development and long-term monitoring of supplies with taste
and odour problems and the results may be correlated with analyses of
traces of organic chemicals in water samples. However, the results of
FPA cannot be readily interpreted for the purposes of assessing
regulatory compliance. Also these approaches involve considerable
investment in resources, training and expertise and are normally only
considered when major taste and odour incidents occur.

Generic guidance is given in the Blue Book and European standards
for the selection of assessment panellists and variation in sensitivity
over time is recognised as a source of error. However, it would be
useful to consider the utility of including positive control samples
into testing regimes using known concentrations of organoleptic
chemicals to get some measure of variation in panellist sensitivities
within batch and between test sessions.

It is important to note that data on water quality from the Drinking
Water Inspectorate indicates that a very small number of water samples
collected from consumers taps fail to comply with the regulatory
dilution number. In apparent contrast to these data is the proportion
of consumer complaints received by DWI which are attributed to taste or
odour problems (33% of 281 complaints received). Obviously, the main
criteria available to consumers judging the quality of drinking water
are visual or sensory and it is in this way that most complaints are
recognised and reported. However, the extensive application of point of
use devices by consumers suggests that current taste and odour
compliance testing may not actually be a representative measure of
acceptability of water to consumers.

The introduction of secondary maximum contaminant levels (SMCLs) in
the USA based on aesthetic effects was intended to ensure that drinking
water was of a satisfactory aesthetic quality regardless of compliance
with health based limits. However there is a need for a definition of
the relationship between threshold limits versus acceptable levels that
are set. The cost implications associated with monitoring for a
significant number of potential organoleptic compounds would be onerous
and would be difficult to justify unless health based limits were also
being exceeded. USEPA SMCLs currently only relate to inorganic
parameters and the consideration of SMCLs for organic chemicals would
need to be focused on those known to cause operational taste or odour
problems.

At present, technologies such as 'electronic tongues' and
'electronic noses' are unlikely to represent a practical alternative to
human sensory screening. However, they may offer a potential means of
detecting changes in overall water 'quality' without giving information
on concentrations of individual chemical species.

Copies of this report may be available as an Acrobat pdf download under the 'Post 2000 Reports' heading on the DWI website.